Mixing in stratified shear turbulence: implications for the Saharan Air Layer

Every year during late spring and summer, the mean easterly winds above Northwestern Africa bring pockets of dust-laden Saharan air to the Americas, causing the Saharan Air Layer (SAL) to form over the tropical North Atlantic. However, gravitational settling of dust particles during their 5-day transatlantic trip in the SAL occurs much slower in reality than current models predict. An explanation for these reduced deposition rates which has been gaining increasing support from models and observations is the occurrence of vertical turbulent mixing within the SAL. Nevertheless, temperature measurements indicate that the SAL interior is usually weakly stratified, so buoyancy effects must be taken into account in order to accurately determine shear-induced turbulent mixing rates. We hypothesize that the initially well-mixed SAL becomes progressively stratified as it moves toward the Americas due to lateral heat fluxes, proportional to the SAL temperature anomaly and resulting from large-scale dynamics. The competition between stratification and turbulent mixing is quantified as the ratio of the corresponding timescales. The impact of this dimensionless parameter on the wind, temperature and dust fields at different Reynolds numbers is assessed via large-eddy simulation (LES) of a stratified shear layer. We aim to link parameters related to the SAL dynamics to mixing rates in stratified shear turbulence, so that turbulence effects on dust fields can be accurately parametrized in large-scale models.